[feature request]: allow iterating over object in comprension (like dict comprehension in python)

It would be useful to be able to do something like dict comprehensions in python, eg: `for key, value in obj`

A use-case would be when defining environment variables for a kubernetes container like so:
```jsonnet
{
env:: {
FOO: 'foo',
BAR: 'bar',
BAZ: 'baz'
},
containerEnv: [{name: k, value: v} for k, v in $.env]
}
```
->
```json
{"containerEnv": [
{"name": "FOO", "value": "foo"},
{"name": "BAR", "value": "bar"},
{"name": "BAZ", "value": "baz"}
]}